A/N: This idea is mostly a Fate/Zero concept, with Potterverse crossover elements. I'm not super well-versed in Nasuverse lore, so I may get some things wrong here, but I welcome corrections. I also welcome people taking my ideas and running with them (hint, hint...)
If you're familiar with the plot/backstory of the Fate series and Fate/Zero in particular, you might be best-served skipping until the first scene-break, as the opening segment is all exposition that mostly sums up salient points from said backstory.
Causality is a funny thing, really. Every decision, every random chance, every event with multiple possible outcomes, creates a branching point in the timeline, resulting in an infinite profusion of parallel realities, no two of which are exactly alike. In one such branch, three families of magic-users, known in their world as Magi, collaborated to craft a ritual intended to grant access to Akasha, the Root of All Things, via an artifact known as the Holy Grail.
This ritual took the form of a deadly tournament, dubbed the Holy Grail War, in which seven Magi would each summon a Servant, a copy of the spirit of a hero or other legendary figure given physical form. Each Servant would fall into one of seven Classes: Archer, Assassin, Berserker, Caster, Lancer, Rider, Saber; how a group of early-nineteenth-century magi came up with an RPG-esque character class system is unknown, though most blame the Wizard Marshal Kischur Zelretch Schweinorg, known to most simply as "Zelretch", "Kaleidoscope" (for the magic he wielded that allowed him to see and interact with other times and realities), or "that undead troll".
When a vampire gets old and bored, sometimes they get a tad strange. Access to the broader multiverse probably didn't help matters.
In any case, over the course of the Holy Grail War, the Master-Servant pairs would battle each other in a winner-takes-all battle royale, with the potential prize starting at "bragging rights" (something most Magi would kill for without batting an eye anyway), ranging through "one wish each to the winning Master and Servant", and topping out at "access to the Root" (and thus infinite knowledge).
With such a prize at stake, and given the total amorality of most Magi, it should therefore come as no surprise that somebody tried to cheat more than usual. During the Third Holy Grail War, the von Einzbern family (one of the three that originally founded the Grail Wars) managed to summon up an extra Servant beyond the normal seven. Unfortunately for the cheaters, their irregular Servant was weak, and quickly defeated.
The reason behind the whole death-tournament was that each Servant, when slain or otherwise knocked out of the contest, would be absorbed by the Grail as fuel. Once all but one Servant had been eliminated, the Grail would be fully-primed and ready to grant a wish, though making a clean sweep (such as by having the winning Master force their Servant to commit suicide) was necessary to grant access to the Root.
When the cheaty Servant was defeated, however, something went wrong. Instead of being absorbed by the Grail as raw mana, the essence of the Heroic Spirit remained intact enough to color the Grail with its own personality forevermore, altering all future Grail Wars accordingly. Given the fact that the Grail itself selected both the Masters and the Servants, this was no small thing even before one reached the point of potential wish-granting.
In the majority of these timelines, the eighth Servant was of the Avenger class, but his true identity was Angra Mainyu, the Zoroastrian devil figure. His unbridled malevolence tainted the Grail, leading to an insane serial killer being chosen as a Master in the Fourth Holy Grail War, his Caster servant the monstrous child-killer Bluebeard. The man that won the Fourth Grail War would see the corruption in the Grail, realize that it would horribly twist any wish and potentially unleash an unstoppable evil upon an unprepared world. He would force his Servant to destroy the manifestation of the Grail, resulting in a catastrophic fire and the deaths of hundreds of innocents. It was an all-around bad time, is what I'm getting at.
Timelines are constantly branching however, and in a small branch off the main trunk of this timeline thing went a bit differently...
"You're giving me a stick." Jubstacheit von Einzbern, known more colloquially as "Acht", glanced impassively between the polished stick of dogwood and the visitor offering it to him.
"Not just a stick," scolded Zelretch, "a magic wand." At his hosts's unimpressed look, he continued. "I picked it up through the Kaleidoscope and thought it could be a good catalyst for summoning a Servant, something I've noticed the von Einzberns haven't done yet this time around."
"We cannot," Acht replied flatly. "Seven Servants have already been summoned."
"They have?!" Zelrecht gasped in blatantly insincere shock. "Well, keep it anyway." He turned to leave, waving airily. "I'm sure you'll find some use for it..."
The irregular summoning ritual completed with a flash of light and a plume of smoke. When visibility returned, it revealed a tall, lean man in tattered robes clutching the so-called "magic wand" that had indeed served as a catalyst. He had shoulder-length black hair, grey eyes, a pale complexion and aristocratic features made somewhat less impressive by his gaunt, somewhat emaciated state and his half-mad expression. "Servant Marauder has answered your summons," he stated with a posh British accent before adopting a manic grin, "and I solemnly swear that I am up to no good."
In his chambers at Clock Tower, Zelretch smiled to himself and whispered, "Mischief Managed."
Sixty Years Later:
The smoke from the summoning cleared, revealing a young man of no more than twenty, though his bespectacled green eyes looked far older. A jagged scar in the shape of a lightning bolt peeked out from behind the fringe of his messy black hair. "Servant Caster has answered your summons," he said, conveying an impressive amount of disgust at the first word, "and if you want me to call you Master it's going to cost you a Command Seal. I should probably warn you, last time I got dragged into a deadly magical tournament it didn't exactly end well..."
"Kiritsugu, darling," Irisviel von Einzbern said with a beatific smile and a saccharine tone that spoke of pain and death in the immediate future if she was not appeased, "please explain to me why your Servant has appeared in nothing but a blue lace negligeé?"
"I was wondering that myself," the Magus Killer replied, keeping his eyes firmly fixed on his wife and not on the blonde woman that somehow managed to look regal and dignified even in skimpy lingere, "can you shed any light on this Saber?"
"I cannot," Arturia stated coldly as she called up her usual armored dress, "though I am thankful that, minimal as it was, my attire when summoned was at least tasteful and preserved my modesty, even if only barely."
Tohsaka Tokiomi gazed in shock at the imperious golden-haired man in his summoning circle. "Servant Archer has answered your summons. I ask of thee," the King of Heroes ground out in a strained voice, "WHY AM I NAKED?!"
Tokiomi decided not to mention the fact that somehow someone had managed to write the word "PONCE" across his new Servant's forehead in what looked like permanent marker. The... anatomical illustration on his cheek was definitely not going to be brought up.
Lancer looked between his Master, who was sliding rapidly from surprise into rage, and the woman that was presumably the man's wife, who was staring hungrily at Lancer's... other spear. "Well," he sighed, "this could have started better."
"Yes, I'm your Master!" shouted a frantic Waver Velvet, "Now please, put on some clothing!"
The bellowing laughter of his newly-summoned Rider servant did not help his already frayed nerves.
Kotomine Kirei gave the figures before him a bland smile. "I'm not sure why you're... attired thus, but please refrain from such debauchery on Church property from now on."
Matou Kariya stared at the hulking armored figure before him, wondering why he felt like some unknown power had taken pity on him...
A/N: Yep, instead of Angra Mainyu turning the Grail evil, Sirius Black has imbued it with a love of pranks and schadenfreude. Those poor, poor Magi...
As an explanation for the first prank of the Grail War, Sirius would most want to humiliate an arrogant prick like Gilgamesh. Harry got passed over for obvious reasons, Lancelot because Sirius figures that Kariya has already been through enough, and Arturia gets at least something to wear out of a combination of what chivalry Sirius possesses and a Briton's respect for King Arthur. She's still a good-looking woman, though, and with Irisviel watching the summoning the temptation was just too much for him to pass up.
Who summoned Harry? I don't know, except that it sure as hell isn't Ryuunosuke. No way would Sirius have picked him as a Master.
Overall, I'm just hoping somebody reads this and is inspired to write their own, more extensive version. I am nowhere near sufficiently well-versed in Fate lore to be able to actually write out a full fic, even if I didn't already have a long-running story in progress. The core concept is stated right at the start of this A/N.